Quantification of the Genetic Expression of bgl-A, bgl, and CspA and Enzymatic Characterization of β-Glucosidases from Shewanella sp. G5

Abstract

Shewanella sp. G5, a psychrotolerant marine bacterium, has a cold-shock protein (CspA) and three β-glucosidases, two of which were classified in the glycosyl hydrolase families 1 and 3 and are encoded by bgl-A and bgl genes, respectively. Shewanella sp. G5 was cultured on Luria-Bertani (LB) and Mineral Medium Brunner (MMB) media with glucose and cellobiose at various temperatures and pH 6 and 8. Relative quantification of the expression levels of all three genes was studied by real-time PCR with the comparative Ct method (2-ΔΔCt) using the gyrB housekeeping gene as a normalizer. Results showed that the genes had remarkably different genetic expression levels under the conditions evaluated, with increased expression of all genes obtained on MMB with cellobiose at 30 °C. Specific growth rate and specific β-glucosidase activity were also determined for all the culture conditions. Shewanella sp. G5 was able to grow on both media at 4 °C, showing the maximum specific growth rate on LB with cellobiose at 37 °C. The specific β-glucosidase activity obtained on MMB with cellobiose at 30 °C was 25 to 50 % higher than for all other conditions. At pH 8, relative activity was 34, 60, and 63 % higher at 30 °C than at 10 °C, with three peaks at 10, 25, and 37 °C on both media. Enzyme activity increased by 61 and 47 % in the presence of Ca2+ and by 24 and 31 % in the presence of Mg2+ on LB and MMB at 30 °C, respectively, but it was totally inhibited by Hg2+, Cu2+, and EDTA. Moreover, this activity was slightly decreased by SDS, Zn2+, and DTT, all at 5 mM. Ethanol (14 % v/v) and glucose (100 mM) also reduced the activity by 63 and 60 %, respectively.